This invention relates to a hydraulic autotensioner for keeping constant the tension of a timing belt for use with an automotive engine.
One known hydraulic autotensioner is shown in FIG. 7. It has a body 1 comprising a cylinder 2 made of a light alloy to reduce the entire weight and a hard sleeve 3 mounted in the cylinder 2. A plunger 4 is slidably mounted in the sleeve 3. A wear ring 6 having a hole 7 that allows the passage of hydraulic oil is press-fitted on a rod 5 which is movable axially together with the plunger 4, and is held in position axially with a shoulder 8 formed on the rod 5. A return spring 9 is mounted between the wear ring 6 and the upper end of the sleeve 3 to bias the rod 5 outwardly.
The interior of the body 1 is partitioned by the plunger 4 into a pressure chamber 10 and a reservoir chamber 11 which communicate with each other through a passage 12 formed in the plunger 4. A check valve 15 is provided near the lower end of the passage 12 leading to the pressure chamber 10.
A retaining ring 14 is fixed to the inner peripheral surface of the body 1 at its upper part to prevent an oil seal 15 having a rod inserting hole from coming out. The oil seal 15 serves to prevent the leakage of hydraulic oil filling the body 1. An air layer is present between the hydraulic oil and the oil seal 15.
In this autotensioner, a pulley arm 16 is pushed up by the rod 5 to press a tension pulley 17 provided at one end of the pulley arm 16 against a belt 18. Thus, a predetermined tension is imparted to a belt 18. The pushing force applied to the rod 5 with variations in tension of the belt 18 is damped as the hydraulic oil in the pressure chamber 10 is compressed by the plunger 4.
As the tension of the belt 18 increases and the rod is pushed in, the check valve 13 is closed and the force transmitted from the belt is borne by the oil pressure. Simultaneously, the hydraulic oil in the pressure chamber 10 is compressed and part of it leaks through a gap between the frictional surfaces of the sleeve 3 and the plunger 4, allowing the rod 5 and the plunger 4 to retract slowly in such a direction as to reduce the tension of the belt.
In contrast, as the belt 18 slackens, the rod 5 and the plunger 4 will move up quickly by the force of the spring 9 in such a direction as to increase the tension of the belt. At the same time, the pressure in the pressure chamber 10 decreases, the check valve 13 opens, and the hydraulic oil in the reservoir chamber 11 flows through the passage 12 into the pressure chamber 10.
In this autotensioner, the tension is applied to the belt 18 by the spring force of the return spring 9. order to increase the force of the return spring and thus the tension of the belt, both the diameter and the entire length of the return spring 9 have to be increased.
In case of the conventional autotensioner shown in FIG. 7, since the return spring 9 is disposed between the sleeve 3 and the wear ring 6, in order to increase the length of the return spring 9, the entire length of the cylinder 2 has to be increased correspondingly. It is difficult to mount an autotensioner having such a long cylinder on an engine.
On the other hand, if the spring constant of the return spring 9 is increased to increase the spring force instead of increasing the length of the cylinder, it will then become difficult to set the spring force of the return spring within a predetermined range, i.e. the range within which the tension of the belt can be kept at a predetermined value.
Generally, an autotensioner is subjected to a high frequency variable load synchronizing with the engine revolving speed. The load is transmitted from the belt Thus, the rod 5 and the plunger 4 will vibrate microscopically at high speed. Since the vibration of the belt 18 is transmitted through the pivotable pulley arm 16, the rod 5 is pushed in not straight but obliquely. Thus, an edge load acts on an edge 19 of the plunger 4 and the edge 19 is likely to get worn. Also, the inner periphery of the sleeve 3 may get worn unevenly by contact with the edge 19, thus inhibiting smooth sliding movement of the plunger 4. Such an autotensioner cannot function properly,